1. Field of the Invention
The invention relates to orthodontic devices, and more particularly, to orthodontic coil springs for mounting to a patient's teeth in order to impart a force to move the teeth.
2. Background Information
In FIGS. 1A and 1B a conventional system for orthodontic treatment is illustrated, wherein each tooth 1 has attached to a surface thereof an orthodontic bracket 2. An archwire 3 is connected to each bracket 2 in order to impart a force to straighten the teeth.
In cases where there is a space between adjacent teeth, such as, for example, the space 4 between teeth 1a and 1b, stop fittings 5 are attached to each bracket 2 of the adjacent teeth. A coil spring is then utilized to connect the two stop fittings and impart a force to move the adjacent teeth into the space 4. In FIG. 1A, a tension spring 6 is shown, having formed on each end hooks 6a which are each connected to a stop fitting 5. The force of the tension spring 6 imparts a force in order to move the teeth 1a and 1b toward one another and into the space 4, as indicated by the arrows in FIG. 1A.
In an alternative system shown in FIG. 1B, a compression spring 7 is fitted over the archwire 3 and between the orthodontic brackets 2 of adjacent teeth 1c and 1d. The force of the compression spring 7 acts to move the tooth 1c away from the tooth 1d and into the space 4, as indicated by the arrow in FIG. 1B.
The known orthodontic coil springs, such as the tension spring 6 or compression spring 7, have been formed of stainless steel. The mechanical properties of stainless steel springs are such that the magnitude of the spring force changes sharply with slight deflections in the spring. In FIG. 2, the deflection-spring force curve for a typical stainless steel tension coil spring is shown X2, and in similar manner, in FIG. 3, the deflection-spring force curve for a typical stainless steel compression coil spring is shown Y2. As can be seen, in both cases, the spring force changes sharply with increasing or decreasing deflection of the spring. Accordingly, one problem associated with conventional stainless steel springs is that after the teeth are caused to move, even slightly, the deflection causes the spring force to decrease and become insufficient to move the teeth as desired.
One method that has been used to compensate for the problem of decreasing spring force in stainless steel springs has been to increase the initial deflection of the spring. As a result, after initial tooth movement the spring can still impart a sufficient force to continue moving the teeth. However, this method has proven to be unsatisfactory since the initial force of the spring is generally too great and as a result, causes pain and discomfort to the patient. Moreover, this method fails to overcome the inherent problem of the relatively sharp changing spring force with deflections in the spring.
One consequence of this problem is that orthodontists have been relegated to frequently changing the stainless steel coil springs so that after there is even slight tooth movement new springs are inserted in order to maintain the optimal spring force against the teeth. This method has proven particularly troublesome in replacing compression coil springs such as spring 7, shown in FIG. 1B. As can be seen, in order to replace the compression spring 7 the archwire 3 must first be disconnected from the orthodontic bracket 2 and then reconnected in order to first remove and then insert a new spring 7.
It is known in the art that the orthodontic forces imparted on one-root front teeth and two-root small molars should be about one quarter to one half of that imparted on three-root large molars. It is required therefore, to correspond the spring force to the type of tooth to be moved. If the spring force is too great for example, the teeth can be improperly rotated or shifted.
The spring force of conventional stainless steel coil springs can only be roughly determined by varying the wire diameter and winding diameter of the spring. Accordingly, one problem associated with conventional stainless steel coil springs is that the optimal spring force necessary for a particular orthodontic application cannot be accurately set for each spring. Moreover, it has not been possible to produce conventional springs that are identical in size yet have different loading characteristics. As a result, it has proven particularly difficult to obtain the optimum spring force to move a particular tooth with conventional stainless steel springs.
Another problem associated particularly with conventional compression coil springs, such as the spring 7 in FIG. 1B, is that because the spring is inserted over the archwire 3, the minimal dimension of the winding diameter of the spring is limited by the diameter of the archwire. The loading characteristics of the spring therefore are also limited as a result of the minimum dimension.
In order to obtain the most suitable spring force with conventional coil springs, the spring is generally trimmed to a length believed to be suitable as determined by measuring the distance of the space between the particular teeth to be moved. Known spring testers or push-pull gauges, not shown, are then utilized in order to set the spring force for the particular size of the space. This process must be repeated with each successive change in springs in order to maintain the optimum spring force for the particular tooth to be moved. Accordingly, the use of conventional orthodontic springs has been particularly time consuming and troublesome.
It is an object of the invention therefore to provide an orthodontic coil spring that overcomes the problems and disadvantages of known coil springs.
It is a further object of the invention to provide an orthodontic coil spring having a preset spring force which is determined for use with a particular tooth to be moved.
It is yet a further object of the invention to provide an orthodontic coil spring for imparting a predetermined and substantially constant spring force throughout a zone of deflection of the spring sufficient to cover the desired movement of a tooth.
Other objects and advantages of the orthodontic coil spring of the invention will become apparent in view of the following detailed description and drawings taken in connection therewith.